Multipart/related with no boundaries and no multipart/related example at documentation

I can't send multipart/related data with boundaries. I know I should not write any content type for multipart for adding boundaries automatically by superagent.

All examples and issues are for multipart/form-data, if I try them with /related they fail.

If I try request.attach or request field ,superagent is adding to those data packages multipart/form-data header automatically,

All similar issues i found out about this:
https://github.com/visionmedia/superagent/issues/746
https://github.com/visionmedia/superagent/issues/1202